QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D ISOLATED DC/DC POWER CONVERTER LT1952 DESCRIPTION Demonstration circuit 968A-D is isolated input to high current output 1/8th Brick footprint converter featuring the LT®1952 switching controller. The DC968-D converts isolated 36V to 75V input to 28V output and provides over 4A of output current. The converter operates at 300kHz with efficiency up to 95%. With proper amount of airflow, the DC968-D converter can generate up to 5A of output current. The DC968 can be easily modified to generate output voltages in the range from 0.6V to 48V. The output currents are limited by total output power of up to 200W. The other available versions of DC968A are: DC968A-A 36-75Vin to 3.3V@30A DC968A-B 18-36Vin to 5V@20A efficiency. Therefore, narrow input voltage range will be more desirable. The DC968 circuit features soft-start which prevents output voltage overshoot on startup or when recovering from overload condition. The DC968 has precise over-current protection circuit that allows for continuous operation under short circuit conditions. The low power dissipation under short circuit conditions insures high reliability even during short circuits. The LT1952 can be synchronized to an external clock of up to 400kHz. Please refer to LT1952 data sheet for design details and applications information. Design files for this circuit board are available. Call the LTC factory. DC968A-C 36-75Vin to 12V@12A Also, the DC968 can be modified for other input voltages like 8V-36V, 18V-75V, 44V-52V, and so on. The wider input voltage range will decrease the converter LT is a trademark of Linear Technology Corporation Table 1. Performance Summary PARAMETER CONDITION VALUE Minimum Input Voltage IOUT = 0A to 4A 36V Maximum Input Voltage IOUT = 0A to 4A 72V VOUT VIN = 36V to 72V, IOUT = 0A to 4A 28V ±3% Typical Output Ripple VOUT VIN = 36V to 72V, IOUT = 0A to 4A 100mVP–P Nominal Switching Frequency 300kHz QUICK START PROCEDURE Demonstration circuit 968 is easy to set up to evaluate the performance of LT1952 circuit. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: on the oscilloscope probe. Measure the input or output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals. See Figure 2. for proper scope probe technique. NOTE: When measuring the input or output voltage ripple, care must be taken to avoid a long ground lead 1. With power off, connect the input power supply to Vin and GND. Make sure that the input power sup- 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D ISOLATED DC/DC POWER CONVERTER ply has sufficient current rating at minimum input voltage for the required output load. 2. Turn on the power at the input. NOTE: Make sure that the input voltage does not exceed 75V. 3. Check for the proper output voltage. Vout = 28V, +/-3%. 4. Once the proper output voltage is established, ad- just the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 5. The DC968 is equipped with an output capacitor CSYS (330uF) that approximates typical system rail capacitance. If system board already has capacitance of similar value CSYS can be removed. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. Figure 1. Proper Measurement Equipment Setup 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D ISOLATED DC/DC POWER CONVERTER Figure 2. Scope Probe Placement for Measuring Input or Output Ripple CHANGING THE OUTPUT VOLTAGE To set the output voltage lower than 28V, change the bottom voltage divider resistor connected to LT4430 FB pin (see the simplified schematic on page 5). For example, to get 24V output, change R38 resistor value to 7.15k. However, keep in mind that changing the transformer as well may increase the efficiency. The schematic on page 6 shows all of the components on the PCB. The schematic on page 6 can be used as a reference if optional circuits need to be implemented. To get higher than 28V output voltages transformer with higher turns ratio may be required. Also, output MOSFETs with higher voltage ratings may be required. Please contact LTC factory for details. PRIMARY MOSFET DRIVER LTC4440 The DC968 has an LTC4440 MOSFET driver U2. The LTC4440 can be disabled if the efficiency is not important. If U2 is disabled, the DELAY pin of LT1952 should be readjusted to optimize the efficiency. Please contact LT factory for assistance. RESET CIRCUIT The DC968 is equipped with an active reset circuit. The active reset circuit is used in order to improve the efficiency by 1% to 2%. If the efficiency is not important, active reset circuit can be disabled. In that case, MOSFET Q2 will need to be replaced with a MOSFET with higher voltage rating. Please contact LTC factory for help in selecting appropriate MOSFET. The active reset circuit does not always help with the efficiency and is disabled on DC968-A, -B and -C demo boards. LT1952 DC968-D 28V @4A Output Efficiency 96% 95% 94% 93% 92% 91% 90% 1 2 Iout [A] 3 4 5 Figure 3. High efficiency of DC968A-D allows the board to be used in thermally critical applications with outputs up to 5A. OUTPUT LOAD STEP RESPONSE The load step response of DC968A-D is very fast even though relatively small amount of output capacitance is present (6.8uF ceramic and 330uF electrolytic). This is thanks to fast error amplifier of LT4430, optimal amount of current slope compensation of LT1952, fast opto coupler and fast error amplifier of LT1952. If higher load steps need to be handled more output capacitance can be added in order to keep the voltage transients at the desired level. The load step transients are shown in Figure 4. 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D ISOLATED DC/DC POWER CONVERTER DEBUGGING AND TESTING The DC968 can easily be tested and debugged by powering the bias circuit with a separate power source. To place DC968 into debug mode remove the resistor R1 and connect 48V, 100mA power source to +Vb node (right side of R1). By doing this, the primary PWM controller LT1952 can be activated without the main primary power being applied to +Vin. Figure 4. Fast transient response of DC968 is superior to many competing power modules without the additional output capacitors. SOFT START FUNCTION The DC968 features LT4430 opto coupler driver that has soft start function which produces monotonic startup ramp shown if Figure 5. The rise time of output voltage is controlled by capacitor C19 that is connected to OC (overshoot control) pin of LT4430. To activate the secondary side MOSFET driver (LTC3900) connect a 10V, 100mA power source to collector of Q9 via 1A, 40V diode. That way the current coming from DC968 bias supply will not be able to flow back to 10V supply once the converter is running. Once the primary and secondary controllers are running the MOSFET gate timing can be checked. If the MOSFET gate timing is correct the main power input can be applied to +Vin. The correct delay from turn on of FG and gate of Q1 is usually in the range from -100ns to +100ns. By slowly increasing the +Vin from 0V to 48V the output voltage and input current can be monitored. The input current should not exceed 100mA without the output load. If one of the MOSFETs is damaged, the input current will exceed 100mA. Figure 5. The LT4430 opto coupler driver produces monotonic output voltage rise at startup without output voltage overshoot. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D ISOLATED DC/DC POWER CONVERTER +Vb R1 +Vin 1.5mH Vu1 2.2R Cin1 3x2.2uF D3 BAS516 +Vr2 R11 82k -Vin D5 PDZ9.1B U2 LTC4440-5 1 C3 1u 2 C21 220p 3 Vcc 2 Q12 BC857BF R15 115k 3 R42 0R 4 5 6 7 13.3k 8 R32 383k R46 2.2k 442k Out R22 R28 C10 0.1u +Vr2 +Vb D12 BAS516 +5V R4710k Comp FB=1.23V Vin Rosc Out Sync PGND Delay Vr=2.5V OC SD Isense GND Blank R33 22K 158k +5V R44 11.3k A1 Vcc B1 RC1 R1 C1 -Q1 Q1 Q2 -Q2 C2 R2 RC2 B2 GND A2 16 15 14 13 12 11 10 9 16 D10 C9 12 10 R23 R9 47R D6 BAS21H Cu1 4.7uF R20 43.2k 100p 11 1.5k R48 150R R16 56k R18 10k 1 2 R21 10k Vaux CG 3 4 C14 1u Vu1 3 C30 100p Boost GND TG Inp TS Sync CS- Timer CG GND Vcc FG 8 Sout 7 6 C8 100p 5 R17 560R T2 PE-68386 C12 470p R19 38.3K C29 0.1u 5 4 BAS516 Vaux R31 10R C16 .22u, 10V C25 47n C23 1uF U7 6 U6 PS2801-1 Sw0 Vu1 R43 1k Vcc CS+ C7 220p D11 R26 220R Vfb 1 FG U5 LTC3900 D9 PDZ9.1B C13 .47u D13 PDZ5.1B R8 2.2R C5 0.22u C11 opt. 9 C28 C27 0.1u 0.1u GND C4 680pF Vu1 2 +5V CG Out Sout 14 13 Csys 330uF, 35V + 5 D8 BAS21H 15 C26 220p Co1 R24 82K R29 U3 HC123 1 2 3 4 5 6 7 8 Sout MaxDC Q3 HAT2174 D1 BAS516 BAT760 1 Vfb Q2 HAT2174 4 TS U1 LT1952-1 R14 33K 10 6 R3 41.2 R13 22K 6 +Vo1 +Vout 5 TG Inp 7 FG Sw0 Q1 PHM21NQ15 Boost GND 1 Rcs1 0.010R Vu1 PA2050.393 2 +Vin R5 10R L1 PA0856.003 6.8uF, X7R +Vin T1 D2 BAS516 L2 Q10 PZTA42 Q13 IRF6217 R45 10k 1 2 3 R30 1.2k U4 LT4430 R27 270R Vcc Opto GND Comp SS C19 1uF, X5R C15 2.2nF, 2kV LTC4440 D14 BAS516 5 FB C17 15n R34 16.9k 6 5 R35 280k +Vo1 4 R38 6.19k C18 opt. C24 opt. QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D ISOLATED DC/DC POWER CONVERTER 6